Mimosoideae is generally recognized as a major subfamily of the angiosperm plant family Fabaceae (alternatively named Leguminosae) and is characterized by flowers with small petals and numerous prominent stamens. Growth forms exhibited by Mimosoideae include trees, shrubs and, less frequently, lianas. Species of Mimosoideae are found in tropical, subtropical and warm temperate regions, to which they are well adapted. In common with members of Fabaceae, they generally form symbiotic relationships with nitrogen-fixing bacteria. Species of Mimosoideae serve as important sources of forage and fuel.
However, certain species of Mimosoideae can be undesirable competitors to plant species beneficial for agricultural and ranch operations. Such Mimosoideae species include huisache and certain mesquite species.
Species of mesquite for which control is often desired include Prosopis glandulosa and P. velutina in the sub-tropics and P. juliflora and P. pallida in the dry tropics. Although these species can grow into trees, they are most often shrub sized. Mesquite is well adapted to semiarid rangeland and pastures, because it can draw water from the water table through its long taproot. Mesquite can also draw water available in surface layers, thereby depriving desirable rangeland grasses of moisture. Even drawing water from the water table can be deleterious, as the proliferation of mesquite in parts of Texas is believed to be partly responsible for lowering groundwater levels (“Mesquite Becoming Thorny Water Issue for All of Texas”, published by Office of Communications and Marketing of Angelo State University, San Angelo, Tex., released Jun. 19, 2001, available Jun. 19, 2009 from http://www.angelo.edu/services/communications marketing/archives/01jun/06-19-01.html). New growth of mesquite has needle-sharp thorns up to 75 mm long that are tough enough to penetrate rubber soles of shoes and even tires (http://en.wikipedia.org/wiki/Mesquite, Jun. 29, 2008). The mesquite species P. glandulosa has spread worldwide and is considered one of the world's worst invasive weeds (“100 of the World's Worst Invasive Alien Species”, published by the Invasive Species Specialist Group of the Species Survival Commission of the World Conservation Union, November 2004, available Jun. 19, 2009 from http://www.issg.org/booklet.pdf).
Mechanically eradicating mesquite is difficult, because the plant's bud regeneration zone can extend 15 cm below ground level; mesquite can also regenerate from a piece of root (“Mesquite” article in Wikipedia as of Jun. 29, 2008, current version available from http://en.wikipedia.org/wiki/Mesquite). Furthermore, control of mesquite with conventional herbicides typically requires high application rates, and even then treatments are often ineffective or only partially effective against established mesquite. Defoliation may be followed by later regrowth. Typically monitoring for most of a year after herbicide treatment is necessary before the plants can be concluded to be completely dead.
Similar to mesquite, huisache (Acacia farnesiana) and certain other Acacia species are invasive, thorny, drought-tolerant trees and shrubs limiting forage in pastures and rangelands. Huisache occurs mainly in southwest Louisiana and southeast Texas in the U.S. Other Acacia species are of worldwide significance as weed pests. A. drepanolobium is considered an invasive savanna weed in parts of Africa. A. karroo is a problem in parts of Africa and Australia. A. catechu and A. nilotica present invasive threats to Australia. As for mesquite, eradication of established Acacia trees and shrubs using herbicides is often difficult, requiring high application rates, and even so, control is frequently incomplete.
PCT Patent Publication WO 2005/063721 discloses a new class of herbicidal pyrimidines, including 6-amino-5-chloro-2-cyclopropyl-4-pyrimidinecarboxylic acid, its esters, thioesters and salts. Although these herbicides have been subsequently found to exhibit herbicidal activity against undesired vegetation in the subfamily Mimosoideae such as Acacia and Prosopis species, their efficacy at relatively low application rates is not always sufficient for satisfactory control of these weeds. Mixtures with certain other herbicides have now been discovered to provide remarkable efficacy, thus affording a particularly useful method for controlling these weeds.